1. Field of the Invention
The present invention relates to novel organically post-treated pigments which are wettable and easily dispersible and methods for producing such pigments. Particularly, the present invention relates to inorganic pigments, such as titanium dioxide, characterized by a twofold organic coating, of which the outer organic layer has amphiphilic properties which improve the pigment wettability and dispersibility and the inner layer, underlying the outer layer, is derived from an organic grinding aid employed when grinding the pigment.
2. Description of Pertinent Art
In addition to forming haze free, high gloss coatings, characteristics of high-grade pigments include good wettability and dispersibility into the medium, such as paint systems, in which the pigments are dispersed. High-grade titanium dioxide pigments commonly have an organic coating which improves their wettability, dispersibility and gloss characteristics when dispersed in a medium such as a paint system. It is known to use glycerine, pentaeryrithritol, trimethylolpropane and triethylolpropane to form organic coatings on either untreated or inorganically coated titanium dioxide pigments for improving the wettability, dispersibility and gloss characteristics of the pigments.
According to German Patent DE 14 67 442, the dispersibility and gloss characteristics of titanium dioxide pigments in paint systems are positively influenced by treatment of the pigments with the reaction products of polyols and ethylene oxide. Surface treatment of titanium dioxide pigments with salts of alkanolamines and hydroxycarboxylic acids also improve dispersibility of the pigments into paint systems.
According to United Kingdom Patent GB 1417574, a mixture of a hydrophilic and a hydrophobic organic material is used as a grinding aid for the pigment in a grinding mill, such as ajet mill. Polyalcohols, carboxylic acids and hydroxycarboxylic acids are cited as hydrophilic organics, while polysiloxane, fatty alchols and dioctyl phthalate are cited as hydrophobic organics.
It is well known to prepare high-grade pigments by grinding titanium dioxide in ajet mill. In most cases, grinding in the presence of steam, as the carrier gas, cannot be dispensed with. Organic materials, such as those described in the paragraph above, are commonly used as grinding aids in the milling process. Such grinding aids improve particle size distribution and transportability, (flowability), of the ground pigment. Also, such grinding aids lower the energy requirements for grinding the pigment.
Pigments are ground in jet mills by a process of attrition. Jet mills have no moving parts. Solid particles, which are to be ground, and a carrier gas are fed into the mill such that the solid particles are suspended in the carrier gas. The internal geometry of the mill imparts a swirling motion to the carrier gas-solid particle suspension such that the solid particles impinge each other and the walls of the mill, thus grinding the particles to smaller size. Organic grinding aids are commonly employed in jet mill grinding operations. The organic grinding aids may be fed into the mill in admixture with the carrier gas, in admixture with the particles to be ground or may be fed separately to the mill.
Titanium dioxide particles are commonly ground to desired pigment particle size ranges in jet mills employing superheated steam as carrier gas. Organic grinding aids, such as those described above, are used to improve particle size distribution and transportability, (flowability), of the ground titanium dioxide and for reducing energy requirements for the grinding process. The organic grinding aids should not be undesirably altered chemically by thermal stress in the milling process. The selection of organic materials is therefore limited by their suitability as grinding aids.
Ground titanium dioxide, from a jet mill, has a surface coating of the organic grinding aids employed in the milling process. This organic coating improves transportability, (flowability), of the ground titanium dioxide. However, organic materials which are most useful as grinding aids are not particularly useful for imparting improved wettability, dispersibility and gloss characteristics to the ground titanium dioxide when it is dispersed as pigment in a medium, such as a paint system, and particularly when the medium is intended to be water dispersible.
Organic materials which are effective for improving wettability, dispersibility and gloss characteristics of titanium dioxide pigments are not particularly effective as grinding aids and are often thermally unstable under conditions within a jet mill employing steam as carrier gas.
Grinding particulate solids in jet mills is well known and widely practiced, as is the use of organic grinding aids in the milling process.
Now, according to the present invention, we have discovered a novel process for preparing high quality ground pigment for dispersal into a carrier medium such as a paint system.
The process of the present invention comprises grinding pigment particles in a jet mill in the presence of a carrier gas and an organic grinding aid, thereby producing a ground pigment of selected particle size distribution and having an organic surface coating, (Organic I), derived from the organic grinding aid. Then, treating the ground pigment with an organic material to form a second organic coating, (Organic II), on the pigment surface. Wherein, the organic grinding aid is selected for improving particle size distribution and transportability, (flow characteristics), of the ground pigment and energy efficiency of the grinding process, without regard to suitability of the organic grinding aid for improving wettability, dispersibility or gloss characteristics of the pigment. And, the organic material selected for the second coating, (Organic II), on the ground pigment is selected for its efficacy in improving wettability, dispersibility and gloss characteristics of the pigment without regard to the organic material""s suitability as a grinding aid, including its chemical stability in the grinding step.
Also, according to the present invention, we have discovered a novel, high-quality pigment comprising a first organic coating, (Organic I), deposited on a pigment particle and a second organic coating, (Organic II), deposited on the first organic coating, which high-quality pigment has improved particle size distribution and improved wettability, dispersibility and gloss characteristics compared to similar pigments of the prior art.
The present invention and its advantages will be described in more detail in the Detailed Description of the Invention, which follows.
Pigments which may be prepared using the methods of the present invention are those which can be effectively ground by jet milling. Jet milling finds utility for grinding materials ranging from hard, refractory materials to soft, temperature sensitive materials. Titanium dioxide, particularly, benefits from jet milling to form pigments having good particle size distribution. The description of the preferred embodiments, which follows, is directed to preparing titanium dioxide pigments, although, it is to be understood that other high-quality pigments may be prepared using the methods of the present invention.
Titanium dioxide suited for processing according to the method of the present invention is prepared by either the sulfate or chloride process and may be in the rutile or anatase form. Titanium dioxide particles fed to ajet mill should be of a size for efficient grinding in the jet mill. As required, titanium dioxide may be subjected to a pregrinding process, (such as sand grinding), for producing titanium dioxide particles suitable for feeding to the jet milling process. Methods for preparing titanium dioxide particles for grinding in a jet mill are well known and widely practiced on a commercial basis.
The titanium dioxide particles may be uncoated or may have an inorganic coating. For providing an inorganic coating, titanium dioxide particles are suspended in an aqueous phase which may contain dispersing aids, such as polyphosphates, aminoalcohols, polyacrylates or inorganic salts. Salts of inorganic oxides, phosphates or silicates are dispersed into the aqueous suspension. The pH of the aqueous suspension is then adjusted for precipitating the corresponding oxide hydrate, phosphate, borate or silicate onto the titanium dioxide particles. Titanium dioxide, with an inorganic coating is recovered by filtration from the aqueous suspension and is dried in preparation for being fed into the jet mill. Methods for providing titanium dioxide particles with an inorganic coating are well known and widely practiced on a commercial basis.
Titanium dioxide particles, (coated or uncoated), a carrier gas and an organic grinding aid are fed into a jet mill where the titanium dioxide is ground to a particle size distribution suitable for use as a pigment in a selected medium, such as a paint system, cosmetic, etc. Jet mills have no moving parts. Within the jet mill, titanium dioxide particles are suspended in the carrier gas and the suspension has a swirling motion imparted to it by the internal geometry of the jet mill. The titanium dioxide particles are ground by attrition between particles and by impingement with the walls of the mill.
Carrier gas provides grinding energy and transport to the solid particles in a jet mill. Selection of the carrier gas for a grinding operation is based upon cost, efficiency of operation and compatibility with the grinding aids and solids being ground. Superheated steam is commonly required as carrier gas for proper grinding of titanium dioxide in a jet mill.
Organic grinding aids are employed to improve particle size distribution and transportability, (flowability), of the titanium dioxide ground in ajet mill and to reduce energy requirements in the grinding process. The grinding aids are fed to the jet mill either in admixture with the titanium dioxide particles or directly, preferably to the carrier gas in the feed to the mill in an amount of about 0.03 wt. % to about 1.0 wt. %, preferably 0.1 wt. % to 0.5 wt. %, based upon the titanium dioxide fed to the jet mill. The grinding aids selected should not be undesirably altered chemically by stresses in the grinding operation, particularly by thermal stresses where steam is employed as carrier gas. The selection of an organic compound is therefore limited by its suitability as a grinding aid. For example, it is known to use: alkanolamines, such as triethanol amine, triisopropylamine, and 2-amino-2-methyl-1-propanol; polyalcohols, such as trimethylolpropane, trimethylolethane, neopentyl glycol and pentaeryrithritol; fatty alcohols; fatty acid esters; and mixtures of hydrophilic and hydrophobic organic compounds, such as polyalchols, carboxylic acids and hydroxycarboxylic acids as hydrophilic compounds and polysiloxane, fatty alcohols and dioctyl phthalate as hydrophobic compounds.
Titanium dioxide particles from a jet mill grinding operation as described above have a surface coating of the organic grinding aid, (Organic I). As the organic compounds are selected for suitability as grinding aids, they are not the most suitable compounds for enhancing wettability, dispersibility and gloss characteristics of the titanium dioxide when incorporated as pigment into a selected medium, such as a paint system.
We have discovered that characteristics of pigment particles can be improved for their intended application, particularly for dispersion in paint systems, by applying a selected organic material to the pigment particles after the grinding process. The organic materials are selected for their capacity to improve desired pigment characteristics, without regard to stability of the organic materials in the grinding process. For example, the wettability, dispersibility and gloss characteristics of pigment particles in their intended medium, such as a paint system, can be improved by applying selected organic materials to pigment particles after the grinding process.
Such organic materials may be applied to the ground pigment by any conventional application method. For example, the organic materials may be applied to the pigment particles in pure form, as aqueous solutions or as aqueous emulsions by methods such as spraying, atomizing or vaporization. For pigment particles having an Organic I layer of organic grinding aid, the selected organic materials are applied to form a second, (Organic II), layer on the pigment particles.
The organic materials are selected to be compatible with both the pigment particles and the medium in which the pigment particles are to be dispersed and for their capacity to improve the desired characteristics of the pigment particles. The hydrophilic/hydrophobic character of the pigment particles can be controlled independently of the grinding aid forming an Organic I layer by controlling the quantity and selecting the HLB, (hydrophilic-lipophilic balance), value of the organic material selected to form the Organic II layer on the pigment particles. For example, an amount of organic material in the range of about 0.1 wt. % to about 1.5 wt. %, based upon weight of pigment, having an HLB value in the range of about 10 to 18, may be selected to form the Organic II layer on the pigment particles.
This method of applying organic materials is well suited for forming an Organic II layer on titanium dioxide pigments having an Organic I layer comprised of organic grinding aid, produced by grinding titanium dioxide in a jet mill in the presence of steam and organic grinding aid. The method, is particularly useful for preparing titanium dioxide pigments intended for use in water-thinable paint systems.
The organic materials are selected for their capacity to improve wettability, dispersibility and gloss characteristics of the titanium dioxide pigments as they are dispersed into the intended, medium, paint system. The organic materials selected are preferably of amphiphilic character and are compatible with both the pigment particles to which they are applied and the medium, paint system, in which the pigment particles are to be dispersed.
Suitable organic materials include non-ionic dispersing and wetting agents having an amphiphilic character for compatibility with the pigment particles and the medium in which the pigment particles are to be dispersed. Particularly suitable for use with titanium dioxide pigments, especially pigments intended for use in water-thinable paint systems, are polyoxyethylenealkenyl ethers or ethoxylated polyalcohols having alkenyl or alkyl residues of from about 6 to 22 carbon atoms and polyoxyethylene residues of from about 3 to 40 oxyethylene units. Preferred are polyoxyethylenealkenyl ethers and ethoxylated polyalcohols having HLB, (hydrophilic/lipophilic balance), values in the range of about 10 to 18 and preferably in the range of about 12 to 14. The quantities of polyoxyethylenealkenyl ethers or ethoxylated polyalcohols used are in the range of about 0.1 wt. % to about 1.5 wt. %, preferably about 0.2 wt. % to 0.8 wt. %, based upon the titanium dioxide pigment.